[sdiy] 4069 vco gives v+ noise?

[Karl Ekdahl]

Ok, the hum is the 50hz non-rectified voltage. I use a
15v AC without center tap, two 4700uF 40v filter caps
(not 6800 as i thought) and a half wave rectifier. 

This drives five voltage regulators (L7805CV (1,5A),
GL7812, GL7815, L7812CV (1,5A), KA7905), two of them
are 1,5A but i don't know about the rest. Maybe i
should change all of them to be 1,5A or maybe i
shouldn't try to drive as many regulators from one
transformer?

Could anyone help me with the math here? How do i
calculate how big filter caps i would need? The load i
will drive would be
"the-average-load-of-a-beginners-first-homemade-small-modular"
:)

/Karl

 --- René_Schmitz <uzs159 at uni-bonn.de> skrev: > Hi
Karl,
> 
> > I use the same pinning as in the schedule you sent
> but it's still 
> > giving noise (in the v+ line). The noise seems
> like a 50hz wave with 
> > some minor disturbances riding on it that seems to
> be the 
> > osciallating tone.
> 
> One quick test if it is related to line frequency is
> to
> set the scope to trigger on the line frequency. (Or
> trigger
> on the AC output of your transformer, if your scope
> has none such thing.)
> 
> > The minor noise can at high frequencies be
> eliminated trough adding a
> >  10uF cap at the v+ line but I can't reduce the
> major noise (the 
> 
> That is possible. There is likely some dirt
> introduced when the CMOS 
> gates switch.
> 
> > probable 50hz) without putting a resistor
> inbetween ground and gnd 
> > pin on the 4069. And that - as i said - makes the
> square out die.
> 
> That naturally reduces the load on the PSU. (Two of
> the six
> gates are used in class A, so they have substantial
> quiescent current.) A 100k resistor reduces the
> current that the 4069
> can draw, and also changes all the biassing. That is
> why the pulse out
> doesn't work anymore.
> 
> > The odd thing is that the noise only occurs if i
> add more modules to 
> > the same powersupply, for example a ASM lfo. The
> powersupply should 
> > be able to drive both modules (at least i think
> so) without any 
> > distrubances. It's based aroun 78xx, 79xx and
> 6800uF caps and uses 
> > the standard layout found in most powersupplies.
> 
> Hmm, 6800uF sounds like a lot. What
> transformers/rectifiers
> do you use? 6800uF would be adequate for a 3amp
> supply.
> You are likely reducing the conduction angle of the
> rectifier,
> if you don't nearly draw that much current. And
> possibly even
> destroying the rectifiers with overcurrent.
> Less is sometimes more. And that certainly goes for
> PSU caps.
> 
> All in all this sounds like the transformer has not
> enough
> margin for the drop over the regulators. (Like a 15V
> type for a 15V PSU....)
> 
> I would try to isolate the problem by using a dummy
> load on your PSU.
> Say use a lightbulb or (hi-power) resistor that
> draws the nominal
> current that the PSU is designed to handle. And then
> you'll see wether
> the regulation is ok or not.
> 
> Cheers,
>   René
> 
> 
> -- 
> uzs159 at uni-bonn.de
> http://www.uni-bonn.de/~uzs159
> 
> 
>  

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